Supersolidity is a quantum state of matter in which both off-diagonal long-range order of a superfluid and crystalline order in a solid coexist simultaneously in same specimen. Its first experimental evidence had been reported by Kim and Chan through a torsional oscillator (TO) experiment. However, it was proposed that a change in a shear modulus of solid helium at low temperatures can be an alternative explanation for the TO anomaly, if the TO is not designed to be rigid. Most previous TOs were made with an improper design in which various non-linear elastic effects mimic the superfluid-like TO responses. For this study, KAIST rigid double-torus TO is constructed to reduce any elastic effects inherent to complicated viscoelastic coupling with TO structures, allowing explicit probe for a genuine supersolid transition. We investigated the frequency- and temperature-dependent TO responses containing solid helium. As a result, we did not find evidence to support the frequency-independent contribution proposed to be a property of supersolid helium. The frequency-dependent contribution which comes from the simple elastic effect of solid helium is essentially responsible for the most of responses. The majority of period drop is linearly proportional to the square of frequency, indicating that the responses observed in this TO are mostly caused by the elastic overshoot effect. We also tested the rigid TO responses under a dc rotation, motivated by the supserolid signature found in rotating solid helium. The KAIST rigid double-torus TO was attached to the RIKEN dilution refrigerator. Most of intriguing results in the previous rotation experiment are not successfully reproduced. Instead, we find that the frequency-independent period drop is suppressed by increasing rotation velocity. This superfluid-like TO responses in rotating solid helium are not able to be reconciled with a framework of elastic stiffening models. We consider several possible scenarios.
초고체는 고체가 초유체성을 띄는 물질의 상태이다. 초고체의 첫 실험적 증거로 비틀림 진동자에서의 공명주기 감소 현상이 보고되었으나, 이후 비틀림 진동자가 단단하게 설계되지 았았을 경우 고체 헬륨의 층밀림 탄성 변화가 같은 현상을 발생시킨 다는 것이 밝혀졌다. 우리는 복잡한 비선형 탄성 효과를 모두 제거한 단단한 이중 비틀림 진동자를 제작하여 다시 초고체를 탐색하였다. 그 결과, 단단한 비틀림 진동자에서 나타나는 공명주기 감소 현상의 대부분은 간단한 선형 탄성 효과인 오버슈트 효과에 의해 발생하였다. 초고체의 증거에 해당하는 주파수에 무관한 신호는 매우 작았다. RIKEN과의 협력연구를 통해 직류 회전이 고체 헬륨에 미치는 영향을 연구했다. 이전 직류 회전 실험에서 발견되었던 초고체 증거들은 대부분 재연되지 않았다. 반면, 회전 속도를 증가시킬수록 주파수에 무관한 공명주기 감소 값이 미세하게 줄어드는 것을 발견했다. 기존의 탄성 변화 모델로는 이 실험 결과를 설명할 수 없다.
